Available evidence suggests that the kidney tubule cell transports cyclic nucleotides via separate influx and efflux systems, both of which may be physiologically important. It is the purpose of this proposal to define these transport systems and to assess whether transport of the second messenger, cyclic AMP, could serve to modulate the response to hormonal first messengers (e.g., parathyroid hormone). Of the cyclic AMP excreted in urine, 20-50 percent results from renal synthesis (nephrogenous cyclic AMP), and this component is under the control of PTH, in that it is greatly reduced in hypoparathyroidism and stimulated by PTH. The nephrogenous component of urinary cyclic AMP in man and hamster is decreased by anion transport inhibitors, suggesting that efflux of cyclic AMP from tubule cells into the urine is a carrier-mediated process. This interpretation is consistent with finding that in cultured cells hormonal stimulation results in carrier-mediated extrusion of cyclic AMP into the medium. Efflux of cyclic AMP may modulate the cellular response to PTH, for in hamster kidney the cellular accumulation of cyclic AMP is enhanced when efflux is blocked. A distinct influx system for circulating cyclic nucleotides accounts for their uptake by kidney; as a result kidney is an important site of cyclic nucleotide metabolism. To confirm that efflux of cyclic AMP into urine is carrier-mediated, I will measure the effects of anion transport inhibitors on the clearance of nephrogenous cyclic AMP in the rat. Using cultured kidney cells, I will further define the extrusion system by measuring the effect of transport inhibitors on extrusion of cyclic AMP, and by covalently labeling the carrier with an analog of cyclic AMP. I will further ascertain whether inhibition of the efflux of cyclic AMP potentiates the cellular effect of PTH. The influx system will be studied in a similar manner, so that its substrate specificity and kinetic characteristics can be defined, and the carrier identified by specific covalent labeling.